Hydraulics & drainage · Open channels & culverts
Open channel flow (Manning) calculator
Discharge, velocity and Froude number for a trapezoidal channel at a given depth.
Standard: Manning's equation
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Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| Bed width | 2 | m |
| Side slope z (H:V) | 1.5 | |
| Flow depth | 0.85 | m |
| Bed slope | 0.002 | m/m |
| Manning n | 0.025 earth channel | |
| Design flow | 3 | m³/s |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| A = (b + zy)y | 2.78 | m² |
| P = b + 2y√(1 + z²) | 5.06 | m |
| R = A/P | 0.550 | m |
| Q = A R^(2/3) S^½/n | 3.34 | m³/s |
| V | 1.20 | m/s |
| Top width | 4.55 | m |
| Froude V/√(gA/T) | 0.490 |
- ✓ Capacity ≥ design flow
- ✓ Subcritical (Fr < 1)
Method, formulas and assumptions
- A = (b + zy)y = (2+1.5*0.85)*0.85 = 2.7837 m²
- P = b + 2y√(1 + z²) = 2+2*0.85*SQRT(1+1.5^2) = 5.0647 m
- R = A/P = 2.7837/5.0647 = 0.54964 m
- Q = A R^(2/3) S^½/n = 2.7837*0.54964^(2/3)*SQRT(0.002)/0.025 = 3.3413 m³/s
- V = 3.3413/2.7837 = 1.2003 m/s
- Top width = 2+2*1.5*0.85 = 4.55 m
- Froude V/√(gA/T) = 1.2003/SQRT(9.81*2.7837/4.55) = 0.48995
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.